Romney had closed in on President Barack Obama on Saturday, turning a 5-point deficit to just 3 points. But Obama still leads Romney 49 percent to 46 percent.

Sunday's Gallup poll — which measures a seven-day rolling average — includes three days of polling after the debate. So the full effect of the debate won't be completely evident until Wednesday.

Notably, Obama's approval rating fell below 50 percent on Sunday to 48 percent, below the "safe" level for an incumbent's re-election. On Thursday, Obama's approval rating had hit its highest point in three years at 54 percent. His disapproval rating was up a point to 46 percent on Sunday.